1. Field of the Invention
This invention relates to a semiconductor integrated circuit having a plurality of blocks of logic circuits which are differently energized, and more specifically, to an improvement for preventing energized block or blocks from both malfunctions and excessive power consumption when the other block or blocks are deenergized.
2. Description of the Related Art
A large-scale integrated circuit has many circuit blocks in a single semiconductor chip. Some of the circuit blocks have to be powered at all times in order to maintain their operating conditions, even if the other blocks are deenergized. An example of the circuit blocks which are always powered is a random access memory (RAM) block. The semiconductor chip has different power supply terminals. One of the power supply terminals is used for the RAM block, the other being used for other logic circuit blocks. Only the power supply through the other power terminals is disconnected with the energized RAM block.
In the conventional large-scale integrated circuits, the RAM block and other logic circuit block or blocks are directly connected by wirings. Therefore, if the logic circuit blocks are deenergized, their output lines become electrically floating. Accordingly, the RAM block receives the unfixed outputs from the deenergized logic circuit blocks as input signals.
Input gates are provided in the RAM block to receive address and/or data input signals. Therefore, if the potentials at the wirings through which the input signals are supplied to the input gates of the RAM block should be in an electrically floating condition, the outputs of the input gates also float electrically in order to make a current flow through the input gates. This current flow continues until the logic circuit blocks are powered, which results in a considerable power consumption. If the large-scale integrated circuit is equipped in a battery-powered equipment, the battery for the RAM block is quickly consumed.
The potential in the electrically floating condition has an uncontrolled value and easily changes its value in accordance with external noise. Therefore, the inputs for the RAM block become arbitrary values which cause the RAM block to loose the memorized contents.